You wouldn't have to enable identd permanently. I didn't suggest to permanently enable it. You would simply have mIRC behave as if "[x] Use ID from email address" was always checked, then remove the global ident userid box. They could still disable ident, but it would always take the id from the (old) email field (which would be called ident field or something). This makes it easier to configure multi-server connections with separate userids rather than having to manually modify the options to make it possible.
The user in that bug report is not depending on the multiple fields out of necessity, but rather because "that's how it's setup". His setup could easily change (as I suggested in that bug report) and he would achieve the same results. In fact, what I suggested was exactly what I'm suggesting now. If mIRC only had one ident field he would not have had that issue in the first place-- his problem actually stemmed from the fact that mIRC was using one field and not the other, and he only wanted it to use one field.
